1. Field of the Invention
This invention relates to wedge type gripping systems for tensile testing and, more particularly, to a gripping element for use in the gripping system for testing materials at elevated temperatures.
2. Description of the Prior Art
The current consensus regarding mechanical tensile/fatigue testing is to utilize a rigid gripping system to provide consistent test specimen alignment from one test to the next. There is also a need to control the gripping forces applied to the tested material so as not to cause it unnecessary damage. During elevated temperature testing, the tested materials change dimensionally prior to and during the test. Dimensional changes occur due to applied mechanical forces and temperature changes. For this reason, the grip pressure also needs to be controlled dynamically to compensate for these dimensional changes.
Commercially available hydraulic activated grips exist but are not designed for operation at high temperatures (in excess of 1000.degree. F. ) during the gripping operation. The high test temperatures are required to test the newer generation materials such as high temperature composite matrices at temperatures as high as 3000.degree. F. This precludes making gripping materials and grips themselves of material sufficient to survive these extreme temperatures. Most hydraulically activated grips are designed to operate at or near room temperature and this limitation creates problems when conducting long term testing on materials at elevated temperatures that exhibit oxidation damage mechanisms. One such grip is the MTS647 Hydraulic Wedge Grip. As a result, the current approach to elevated tensile and fatigue testing incorporates a "cold grip configuration" where the center of the specimen is heated and the grip region is outside the hot environment. This test configuration presents several potential problems: (1) large thermal gradients and thermal stresses are produced in the specimen which contributes to failure; (2) materials which exhibit an increased strength at elevated temperatures fail outside the test environment region and at the grips; and (3) protective environmental coatings which tend to work over a limited temperature range do not protect the entire specimen between the hot section of the cold grips. This produces failures outside the test environment region.
To overcome these problems, it has become necessary to increase the temperature of the tested material within the grip region, such as a modified wedge that extends outside the grip body to allow for heating of the wedge itself by either surrounding the extended wedge segment with resistant heating coils or with an inductive heating coil. However, the following problems exist with this arrangement. (1) Acceptable thermal profiles on the test specimen are impossible to achieve due to the tremendous heat flow from the wedges into the grip body and/or cooling water; (2) applying gripping forces by means of an unconstrained wedge becomes extremely difficult to achieve structurally, particularly at elevated temperatures; (3) by extending the wedge beyond the confines of the grip body, wedge material is subjected to a complex, stress state of bending, compression, tension and shear forces that will be further debilitating when operated at elevated temperatures; (4) current art does not provide heat insulating materials to combat heat transfer from the heated wedge area into the grip body, thus requiring an increase in unneeded amounts of heat energy to compensate for losses into the grip body and accordingly, increase the grip body temperature and the cooling capacity needs of the grip body; (5) extending the wedge beyond the grip body will require a longer load train; and (6) if the heating of the wedges are to be accomplished using radio frequency (RF) induction energy then it will require the use of two expensive RF generators and the additional utilities needed to support and operate them.
Accordingly, it is an object of the present invention to provide a wedge for a grip that is capable of being heated and overcomes the prior art problems.